A carbon nanotube (hereinafter may be referred to as “CNT”) is a material having a structure formed by rolling in a tube shape a graphene sheet in which carbon atoms are arranged in a hexagonal, mesh-like pattern. Since the carbon nanotube has a large surface area, an extremely strong mechanical property, an excellent electronic property, and the like, it is expected to be applied to various fields. As the CNT, there are a single-wall CNT formed by rolling one graphene sheet and a multi-wall CNT formed by rolling a plurality of graphene sheets. The CNTs show different properties depending on whether the CNT is the single-wall CNT or the multi-wall CNT, and chirality which changes by how the graphene sheet is rolled. It is known that the CNT shows semiconductivity or metallicity depending on the chirality, and a ratio of the CNTs showing the semiconductivity to the CNTs showing the metallicity is 2:1. As described above, the chirality depends on how the graphene sheet is rolled, and relates to the diameter of the CNT. Therefore, controlling the diameter of the CNT leads to controlling the property of the CNT, so that controlling the diameter of the CNT is extremely important when synthesizing the CNT.
In general, as methods for synthesizing the CNT, there are (1) Arc Discharge, (2) Laser Vaporization, and (3) Chemical Vapor Deposition (hereinafter referred to as “CVD”) (see Nonpatent Document 1, page 23 to 40 for example). As the CVD, there are a thermal CVD for thermally decomposing a material gas and a plasma CVD for decomposing the material gas by plasma. The outline, advantages, and disadvantages of each synthesizing method are as follows.
Arc Discharge is a method for synthesizing the CNT by carrying out an arc discharge using a carbon rod as an electrode in a vacuum or in an inactive gas atmosphere. Arc Discharge has the advantage that the crystallinity of a graphite layer of the CNT obtained by Arc Discharge is excellent, and the disadvantage that the amount of impurities, such as amorphous carbon, of the CNT obtained by Arc Discharge is large.
Laser Vaporization is a method for synthesizing the CNT in a process of sublimating carbon by irradiating a carbon rod with laser in an atmosphere of a noble gas, such as argon. The single-wall CNT can be selectively obtained by using Laser Vaporization. Laser Vaporization has the advantages that the crystallinity of the single-wall CNT obtained by Laser Vaporization is excellent and it is useful for finding out the mechanism of synthesizing the nanotube, and the disadvantages that the amount of synthesis is small, it does not suit for mass-production, and the multi-wall CNT cannot be synthesized.
The CVD is a method for synthesizing the CNT by supplying as a carbon source a hydrocarbon gas, such as methane or acetylene, or an alcohol, such as methanol, to a chamber and directly decomposing the carbon source on a substrate mounting catalyst particles. As described above, there are two major types of the CVD, the thermal CVD and the plasma CVD. The CVD has the advantages that, for example, the purity of the CNT is high, it is suitable for industry since it can realize mass-production, and it can synthesize the CNT directly on the substrate. Further, the CVD has the advantage that since the CNT is synthesized using the catalyst particles on the substrate, the diameter of the CNT can be arbitrarily controlled by controlling the diameter of the catalyst particle. The CVD has the disadvantage that, for example, the crystallinity of the multi-wall CNT is low.
Among the above three methods, only the CVD can directly synthesize the CNT on the substrate. If the CNT can be directly synthesized on the substrate, devices can be directly manufactured, such as synthesizing the CNT on an integrated circuit. Therefore, the CVD using the catalyst particles has been attracting attention.
To directly synthesize the CNT on the substrate, catalyst metal particles need to be formed on the substrate. A general method for forming the catalyst metal particles are deposition of catalyst metal and a method using catalyst metal colloidal particles. How to deposit the catalyst metal and optimization of the diameter of the catalyst metal particle are important factors in synthesizing the CNT, and various known-howl are required.
In the synthesis by the CVD, a CVD process is carried out after the catalyst metal particles are formed. In many cases, the temperature of the CVD process is high, such as 600° C. or higher. In this case, a problem arises where the catalyst metal particles formed on the substrate agglomerate by heat of the synthesis of the CNT. It is generally known that the diameter of the CNT becomes the diameter of the catalyst metal particle. Therefore, if the catalyst metal particle increases in diameter by the agglomeration, the CNT also increases in diameter.
To prevent the agglomeration of the catalyst metal particles, a method using the substrate mounting a catalyst supporting material is known. This method is a technology which prevents the agglomeration of the catalyst metal particles by placing the catalyst metal in nanopores of the catalyst supporting material, keeps small diameters of the catalyst particles, and thus synthesizes the CNTs having small diameters. In Patent Document 1 for example, zeolite is used as the catalyst supporting material. Zeolite is a material having a plurality of fine through holes. By placing the catalyst metal at the bottom of the through holes of the zeolite layer, the agglomeration of the catalyst metal particles in the CVD process is prevented, and the CNTs having small diameters are synthesized.
Further, Nonpatent Document 1 describes on page 269 to 278 that an aluminum anode oxide film is used as a die of the CNT synthesis.
Moreover, when directly synthesizing the CNT on the substrate, an adhesive strength between the substrate and the CNT depends mainly on a contact point between the substrate and the CNT or a contact point between the substrate and the catalyst.
Normally, the diameter of the CNT and the diameter of the catalyst are on the order of nanometers, and the adhesion is maintained only by an area formed by the diameter of nanometers. Therefore, the adhesive strength between the substrate and the CNT is low in many cases, and peeling of the CNT after synthesizing the CNT becomes a problem in many cases.    Patent Document 1: Japanese Laid-Open Patent Application Publication 2003-335509    Nonpatent Document 1: “Fundamentals and Applications of Carbon Nanotubes” written by Riichiro Saito and Hisanori Shinohara and published on March, 2004 by BAIFUKAN CO., LTD